Extruded high-voltage cables utilized thick layers of insulation to ensure reliable service life. Such cable designs lead to challenges in cable manufacturing including the following: i) sufficiently long vulcanization processes to ensure adequate crosslinking of the inner layers of the insulation, ii) sufficient cooling process to cool the cable to enable reeling, iii) controlled cooling to minimize longitudinal stresses leading to “shrink-back” of the conductor from the insulating layers, iv) difficulty in cable centering in some manufacturing configurations in which heavy-wall cable designs are subjected to gravitational forces that lead to sag of the molten insulation around the conductor, v) long degassing times required to remove crosslinking byproducts via a diffusion process through thick layers of insulation, and vi) limited availability of cable lines suitable for high-voltage cable manufacturing.
The state-of-the-art cable manufacturing process involves a true-triple extrusion of an insulation layer between two semiconductor (shielding) layers in a concentric fashion around the conductor. This process provides smooth interfaces between the insulation and the surrounding materials and avoids the introduction of contamination in a multiple-step process. However, for thick insulation layers thermally induced crosslinking and subsequent evacuation or degassing of crosslinking byproducts leads to low productivity.
Although the potential for extruding the insulation layer in multiple steps is known, no reference suggests a multiple extruded insulation layer that is separated by an intermediate layer of high conductivity and/or permittivity. The multiplicity of layers in the absence of such an intermediate layer leaves open the potential for introduction of contamination or voids between the mating insulation layers, which would negatively impact cable reliability. Moreover, since extruding the insulation layer in multiple steps would require the steps to be performed in relatively rapid succession so that the second semiconductor layer can be applied to allow collection and storage of the cable, the advantage of reduced crosslinking and degassing times due to the curing of thinner layers is lost since the insulation layer is just as thick as if it had been extruded in a single step. Still further, use of multiple extrusion layers in a single pass process would require existing manufacturing processes to install an additional extruder in an attempt to achieve the quality that is currently achieved in the state-of-the-art true-triple process.